BACKGROUND: The use of brain imaging to study the
central control of the sympathetic nervous system in human subjects has largely
relied on correlating changes in signal intensity with indirect measures of
sympathetic outflow. We recently showed that it is possible to use intraneural
microelectrodes to record muscle sympathetic nerve activity while performing
functional magnetic resonance imaging (fMRI) of the brainstem (Macefield &
Henderson, 2010). We have since gone on to identify suprabulbar areas within
the brain that may contribute to the generation of spontaneous MSNA and SSNA at
rest. METHODS: SSNA was
recorded via a tungsten microelectrode in 13 subjects. Gradient echo,
echo-planar fMRI was performed using a 3T scanner (Philips Achieva). 200
volumes (46 axial slices, TR=8 s,TE=40 ms, flip angle=90
deg, rawvoxel size
=1.5x1.5x2.75 mm) were collected in a 4s-ON, 4s-OFF protocol. Total sympathetic
burst amplitudes were measured from the RMS-processed mean voltage amplitude
during the 4 s period between scans. Blood Oxygen Level Dependent (BOLD) changes in
brain signal intensity (SPM5: random
effects, uncorrected p<0.005) were measured during the subsequent 4 s
period. RESULTS: Spontaneous SSNA
was positively correlated to signal intensity in the right anterior insula,
right frontal cortex, right orbitofontal cortex and bilaterally in the
mid-cingulate cortex and precuneus. Conversely, the left orbitofrontal cortex
and left anterior insula was negatively correlated to signal intensity and skin
sympathetic nerve activity. CONCLUSIONS: In conclusion, by performing concurrent microneurography and fMRI we have shown that spontaneous
fluctuations in sympathetic nerve activity to skin covaries with activity in
several discrete areas within the brain, leading us to conclude that certain
cortical areas, such as the precuneus, may provide a “wakefulness drive” to
SSNA.